I think what a lot of people misunderstand about why age-gaps and/or p*doph*lia is wrong is that it's not the number difference itself that's the problem, but the maturity and comprehension difference.
A 25-year-old dating a 30-year-old or a 42-year-old dating a 55-year-old is not like someone in their early 20s dating someone in their 40s, and all of those are NOTHING LIKE a 15-year-old involved with a 21-year-old.
A healthy relationship requires equal control, the ability for all involved to be on even footing power-wise, and someone who's just newly an adult can't have that with someone who's already been grown for most of their life. And it's completely impossible for a child because how they automatically put authority in those older than them, even if it's just a year or two. Even a 15-year-old with a 17-year-old is sketchy.
Maturity, comprehension, authority, a developed sense of self; an age-gap where one of the people involved doesn't have all these is when it's unacceptable. It's the same reasons bestiality is wrong. It isn't about the math, it's about personal development.